Negative values of log transformed backscattered values

I followed the several steps for preprocessing of Sentinel-1 images as outlined, ) orbit file application to SAR images; ii) radiometric calibration; iii) speckle filter application; iv) terrain correction; v) conversion of unit-less backscatter coefficient to decibel.
After all these steps the final values are negative. Is there any reason for only negative values?
Thank you in advance.

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after conversion to dB, 0 represents a fully isotropic scatterer. Values below zero indicate that only a part of the transmitted signal has returned to the sensor.

Maybe this also helps you: What is a decibel and how can negative values exist?

Image source

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